Field of the Invention
The present invention relates to a water heater such as a gas water heater.
Description of the Related Art
Specific example of a water heater is a latent heat recovery type shown in FIG. 11A. In a water heater WH shown in FIG. 11A, water is heated by recovering sensible heat and latent heat from combustion gas generated by a burner 5 using a primary heat exchanger HE1 and a secondary heat exchanger HE2. Water supplied to an inflow port for water 20 passes through an inflow path for water 2, flows to be heated in a second heat transfer tube T2 of the secondary heat exchanger HE2, then passes through an intermediate flow path 3, flows to be heated in a first heat transfer tube T1 of the primary heat exchanger HE1, and passes through an outflow path for hot water 4, thereby reaching an outflow port for hot water 40. The outflow path 4 and the inflow path 2 are connected via a bypass flow path 21 and the temperature of outflowing water is controllable at a target value by controlling the amount of water to be flown into the outflow path 4 through the bypass flow path 21 from the inflow path 2.
In the water heater WH, when hot water supply is restarted sometime after being stopped, there is a disadvantage such that the temperature of water (the temperature of the water flown from the outflow port 40) temporally becomes lower than a target temperature. Such a disadvantage is caused when a natural convection of water shown with the arrows N1 to N5 in FIG. 11B is generated at the time of stopping hot water supply. Such a natural convection is caused by the following principle. When hot water supply is stopped, the water in the first heat transfer tube T1 passes through the intermediate flow path 3 and goes up toward the second heat transfer tube T2 because the water in the first heat transfer tube T1 has a higher temperature and a smaller specific gravity than the water in the second heat transfer tube T2. Then, the water in the second heat transfer tube T2 flows toward the inflow path 2. When water flows into the inflow path 2, the water which is not heated and has a low temperature in the bypass flow path 21 flows into the outflow path 4. When hot water supply is restarted under such a condition, water which is not heated and at a low temperature and which has flown into the outflow path 4 from the bypass flow path 21 reaches the outflow port 40 and is supplied to a predetermined destination. As a result, there is a worry that the water having a relatively lower temperature than a target temperature is supplied to the destination.
It is desirable that such a disadvantage is appropriately prevented. In the conventional art, a trap flow path 99 is provided for the intermediate flow path 3 connecting the first and the second heat transfer tubes T1, T2 as shown in FIG. 11C (referring to Patent Literatures 1 and 2). In such a case, the trap flow path 99 inhibits the water in the first heat transfer tube T1 from going up toward the second heat transfer tube T2 at the time of stopping hot water supply. Therefore, “natural convection of water” as mentioned above is inhibited.
There is a case that the water in the first heat transfer tube T1 is not adequately prevented from going up toward the second heat transfer tube T2 only by providing the trap flow path 99 for the intermediate flow path 3. The applicant of the present invention has already proposed a first heat transfer tube provided in a single stage in the vertically height direction and in a meandering manner in plan view as disclosed in Patent Literature 3. However, in such a case, it is further difficult to prevent the above-mentioned natural convection. It is because that the temperature of the water in an upper tube is lower than that in a lower tube when the first heat transfer tube T1 is constituted with two tube bodies up and down as shown in FIG. 11A to FIG. 11C, so that the natural convection shown in FIG. 11B is slightly weakened; on the other hand, such a situation does not occur when the first heat transfer tube T1, meandering in plan view, is provided vertically in a single stage as mentioned above.